In the rest of the present description and in the following claims, the expression “toothed wheel assembly” is meant to indicate the assembly of at least two toothed wheels configured to make at least one part of the sprocket assembly for a rear wheel of a bicycle.
The toothed wheel assembly is intended to be mounted on a freewheel body of a hub of a rear wheel of a bicycle to be engaged by a transmission chain.
As known, the motion transmission system of a bicycle comprises a pair of pedal cranks, on which the cyclist exerts a propulsive thrust, one or more driving toothed wheels, set in rotation by direct coupling with the pedal cranks, and a plurality of driven toothed wheels, also called sprockets, of different sizes, set in rotation by the driving toothed wheels through a chain.
The sprockets are coupled with the rear wheel of the bicycle through a hub. This comprises a first body rigidly connected to the rim of the bicycle through spokes, and a second body rigidly coupled with the sprockets and capable of rotating free with respect to the first body in one direction of rotation, and of setting it in in rotation in the opposite direction, thus causing the advancing movement of the rear drive wheel. In the technical jargon, this second body is called “freewheel body”. The totality of the sprockets, or toothed wheels, mounted on the freewheel body is commonly called a “sprocket assembly”.
The chain consists of a succession of links, each normally consisting of a pair of plates facing one another and spaced apart to define an insertion space of a tooth of a toothed wheel. The plates of a link are rotatably coupled with the plates of the next link through a rivet, which is in turn surrounded by a roller or bush capable of rotating freely with respect to the rivet.
The sprockets in general are made of extremely strong material (like steel or titanium) to withstand the wearing generated by the sliding of the chain and the loads to which they are subjected during operation.
The sprockets of the sprocket assembly can comprise a radially inner annular portion provided with grooves adapted for engaging the grooves of the freewheel body. In this case, the sprockets are axially spaced apart by spacers that are usually cylindrical in shape.
Alternatively, the sprockets of the sprocket assembly can be connected to one another to form a toothed wheel assembly supported by a cylindrical element having a radially inner surface provided with grooves adapted for engaging the grooves of the freewheel body.
In this last case, the sprockets connected to one another axially can be two or more, even up to the totality of the sprockets that make the sprocket assembly.
Document US 2012/0302384A1 and document US 2009/0215566A1 show respective sprocket assemblies entirely formed from sprockets axially connected to one another and connected to a support element for engaging the freewheel body.
Document U.S. Pat. No. 4,380,445 shows a toothed wheel assembly comprising two sprockets, which makes a so-called twin-set, said assembly being able to be associated with further toothed wheel assemblies to define the sprocket assembly.
In a toothed wheel assembly, the axial connection between the sprockets has the dual function of keeping the sprockets axially spaced apart and of mechanically connecting them to one another to give the necessary mechanical strength to the sprocket assembly.
Such an axial connection can be made by cylindrical portions connected to a side surface of a first sprocket and to the side surface of the adjacent sprocket directly facing the first sprocket.
However, in order to reduce the weight of the sprocket assembly, it is common practice to make openings in the cylindrical portions of the axial connection, as described for example in document US 2012/0302384A1 cited above.
In order to keep the weight of the sprocket assembly as low as possible, it is also usual to make the sprockets of the toothed wheel assembly with radial dimensions that are as small as possible. In this way, the axial connection between the sprockets is made in a radially inner portion of the sprocket immediately below the teeth of the sprocket, as shown for example in document US 2009/0215566A1.
The Applicant has noted that the toothed wheel assemblies for a sprocket assembly can cause instability of engagement of the transmission chain during use, especially when intensive, of the bicycle.
The Applicant has found that dirt, small debris and similar can slip between two sprockets and deposit on the connection portions between the sprockets.
The Applicant has noted that since such connection portions are close to the teeth of the sprocket (to reduce the weight of the sprocket assembly), the accumulation of dirt and/or small debris can be such as to reach the pair of mutually facing plates of the chain defining the insertion space of the tooth of the toothed wheel.
The Applicant has found that in such a situation the tooth of the toothed wheel could not be completely engaged by the insertion space, since the pair of plates tends to rest on the accumulation of dirt and/or small debris, actually lifting the chain with respect to the tooth.